Vou subir ao quarto para buscar o meu caderno.

Portuguese often leaves subject pronouns out because the verb ending already shows who the subject is. Here, vou already tells you it means I go / I’m going.

So Vou subir... is perfectly normal.
You would add eu mainly for emphasis, contrast, or clarity:

Eu vou subir, mas tu ficas aqui.
= I’m going upstairs, but you stay here.

Vou subir uses the very common pattern ir + infinitive, which often expresses a near future or an intention.

So vou subir can mean things like:

• I’m going to go up
• I’m going upstairs
• I’ll go up

You could also say Subo ao quarto..., but that sounds different. Vou subir feels more like an immediate plan or decision, which is very natural in conversation.

Because subir means to go up or to go upstairs. It adds the idea of upward movement.

• Vou ao quarto = I’m going to the bedroom
• Vou subir ao quarto = I’m going up to the bedroom

So subir tells you the room is on a higher floor, or at least that the movement is upward.

Ao is a contraction of a + o.

• a = a preposition, often meaning to
• o = the (masculine singular)

So:

a + o = ao

That means ao quarto literally means to the room / to the bedroom.

A useful comparison:

• ao = to the for masculine singular nouns
• à = to the for feminine singular nouns (a + a = à)

Because ao quarto shows movement toward the room, while no quarto shows location inside the room.

• ao quarto = to the bedroom
• no quarto = in the bedroom

So in this sentence, the speaker is moving somewhere, not already there.

Yes, para o quarto is possible. Both can mean to the bedroom.

But there is a slight difference in feel:

• ao quarto is a very natural destination pattern with verbs of movement
• para o quarto can sound a bit more directional, like towards / headed for the bedroom

With subir, subir ao quarto is very idiomatic and natural.

Para + infinitive is a very common way to express purpose in Portuguese. It means to or in order to.

So:

para buscar o meu caderno
= to fetch/get my notebook

It answers the question why the speaker is going upstairs.

Another example:

Fui à cozinha para beber água.
= I went to the kitchen to drink water.

It is correct and understood, but in European Portuguese many speakers would more naturally say ir buscar when they mean go and get / fetch.

So in Portugal, a very natural version would be:

Vou subir ao quarto para ir buscar o meu caderno.

The original sentence is still understandable and grammatical, but ir buscar often sounds more everyday in European Portuguese.

Also, don’t confuse buscar with procurar:

• buscar = fetch / go get
• procurar = look for / search for

In European Portuguese, possessives normally take the definite article.

So the usual pattern is:

• o meu
• a minha
• os meus
• as minhas

That is why o meu caderno is the normal European Portuguese form.

Using meu caderno without the article is much less typical in Portugal, although learners may notice it more in other varieties of Portuguese.

In everyday Portuguese, quarto usually means bedroom.

So ao quarto will normally be understood as to the bedroom, especially in a sentence like this. English sometimes says room, but in Portuguese quarto is usually more specific than that.

If you want to talk about a room in a more general sense, Portuguese often uses a different word depending on the context.